This master thesis focuses on fan subculture and specifically on the cultural phenomenon of fanfiction, a type of literary fanwork. The phenomenon is defined and put into cultural and historical context. The next part of the thesis outlines the history of fandom, fan studies and lists the most influential works in the field. In the following part, fandom terminology is explained. The analytical part of the work focuses on fanfiction written about the British literary saga Harry Potter, written by Joanne K. Rowling. The analysis examines the metamorphosis of relationship between Harry Potter and Severus Snape in fanfiction as opposed to its depiction in the source narrative.